the ntp engine can run with "stdio inet proc". For many reasons,

including fork/exec cost, it would be better if constraints were
forked from the master process, which would then tell the ntp
engine.  That would increase accuracy and security.
Lots of conversations with reyk and bcook

src/usr.sbin/ntpd/ntp.c

@@ -1,4 +1,4 @@
-/*	$OpenBSD: ntp.c,v 1.135 2015/08/14 02:00:18 millert Exp $ */
+/*	$OpenBSD: ntp.c,v 1.136 2015/10/09 03:54:53 deraadt Exp $ */
 
 /*
  * Copyright (c) 2003, 2004 Henning Brauer <henning@openbsd.org>
@@ -30,6 +30,7 @@
 #include <string.h>
 #include <time.h>
 #include <unistd.h>
+#include <err.h>
 #include <tls.h>
 
 #include "ntpd.h"
@@ -165,6 +166,20 @@ ntp_main(int pipe_prnt[2], int fd_ctl, struct ntpd_conf *nconf,
 
 	endservent();
 
+	/*
+	 * XXX
+	 * Unfortunately, the "contraint" processes are forked
+	 * below the "ntp engine".  Hence the ntp engine needs
+	 * to be able to fork -> "proc", and the "constraint"
+	 * process will want to open sockets -> "inet".
+	 *
+	 * For many reasons, including fork/exec cost, it would
+	 * be better if constraints were forked from the master
+	 * process, which would then tell the ntp engine.
+	 */
+	if (pledge("stdio inet proc", NULL) == -1)
+		err(1, "pledge");
+
 	signal(SIGTERM, ntp_sighdlr);
 	signal(SIGINT, ntp_sighdlr);
 	signal(SIGINFO, ntp_sighdlr);